The present invention relates to a heater for an indirectly-heated cathode and, more particularly, to the structure of a coil winding of a cathode heater.
This kind of cathode heater is used as, for example, a thermionic emission heater which constitutes an electron gun of a color picture tube, and has what is called a duplex winding structure in which a core wire having tungsten as the main constituent is wound around a molybdenum mandrel into a coil as a primary winding and the coil is further coiled into a secondary winding. Such a structure of a heater for indirectly-heated cathode is well known, and is described in, for example, Japanese Patent Provisional Publication No. 50564/'76.
However, in the primary winding of a cathode heater in which a molybdenum mandrel is utilized, the diameter of the molybdenum mandrel is ordinarily 1.9 to 4.0 times that of the tungsten core wire, the diameter of the mandrel being ordinarily the same as the inner diameter of the primary winding. As a result, when the tungsten core wire is wound around the molybdenum mandrel so as to constitute the first winding, compression stress is applied to the inward of the winding direction of the tungsten core wire and tensile stress is applied to the outward of the winding direction thereof. The smaller the ratio of the diameter of the molybdenum mandrel to that of the tungsten core wire, the larger become these stresses. If these stresses are large, the mechanical strength of a heater constituted by the winding is disadvantageously so reduced as to generate breaking of wire.